There are two categories of insulators made of organic materials, namely, firstly, line insulators, which are subjected to stresses which may be either tensile or embedded bending stresses, and secondly equipment insulators which are subjected to stresses which may be either compression or embedded bending stresses. With both these categories of insulators, it is always of prime importance to attenuate the surface arcing phenomenon which is frequently encountered with apparatus used in highly-polluted zones.
Indeed, as in known glass or porcelain insulators, the surface arcing phenomenon occurs in line or equipment insulators used in highly polluted zones. Such a phenomenon is related to a damp layer of conductive polluting substance on the surface of the insulator: the leakage current dries said layer in some high current density zones and conditions thus promote the generation of electric arcs which short-circuit the dry zones.
Depending on the type of insulator used, numerous solutions have been proposed to mitigate the surface arcing phenomenon. They are generally based on the principle of providing a semiconductor zone between two electrodes so as to modify the distribution of the electric field in such a way as to make it less favourable to the generation of surface arcs.
In the case of know inorganic insulators, it has been proposed to provide surface coatings of enamel containing oxides of iron, titanium or tin; these coatings provide better electronic conduction. In practice, there has often been a great difficulty in providing good-quality and durable bonding between the insulating material and the semiconductor material. It has also been proposed to provide outer casings impregnated with oxides of iron, titanium or tin, or casings impregnated with graphite powder or with carbon black, for line insulators made of organic material. This applies in particular for those which include a bar of glass fibres impregnated with epoxy resin, said bar being covered with a casing which includes fins and whose function is both to protect the bar and to lengthen the leakage path (such a bar providing high tensile strength together with light weight).
However, the proposed solutions, although technically interesting, remain very difficult to put into practice with insulators for outdoor use. Indeed, drawbacks are often due to an electrochemical corrosion phenomenon, in particular in contact with the electrodes.
Interesting solutions (e.g. French Pat. No. 2,292,318) have also been proposed for equipment insulators made of organic material and in particular for those constituted by a body of moulded resin (generally based on those constituted by a body of moulded resin (generally based on epoxy-cycloaliphatic resin)) with electrodes or metal end pieces at least partially sunk into the ends of said body. The electrodes or end pieces are electrically connected by a thin resistor rod incorporated in the insulating body or by conductive material impregnated in the resin so that a small heating current leaks across the insulator. Indeed, there have been proposed casings of the same type as those mentioned hereinabove, having semiconductor materials distributed throughout their mass or having parts whose central portions only are semiconductive.
However, although these solutions may be suitable for equipment insulators, they are not directly applicable to line insulators such as those to which the present invention relates, because of the absolute necessity for high tensile strength which can not be provided by a resin body with end pieces sunk therein following a design suitable for withstanding compression stresses.
The present invention aims to provide a line insulator whose constitution withstands the surface arcing phenomenon when used outdoors, while remaining simple in design and relatively easy to manufacture.